The present invention relates to ambient temperature correction for liquid expansion thermometers with remote reading transmission.
Such thermometers are currently composed of three sections: a bulb located within the medium whose temperature is to be measured, a duct having a length at least equal to the distance at which the reading of the measured temperature is to be forwarded, and a measurement system connected to the display system of the temperature to which the bulb is subjected. The assembly is filled with a liquid of known coefficient of expansion.
The liquid contained in the duct is subjected to the ambient temperature and its volume will vary as a function of the latter; thus, errors in the indication of the temperature of the bulb will result. A means of correction for such errors is required.
A known means of correction for such errors involve introducing into the duct and over its whole length a wire of a very low expansion material such as "Unvar". In accordance with a particular prior art technique, wherein the expansible liquid in the thermometer is mercury, the inner diameter of the duct is about 1.35 mms. and the diameter of the "Invar" wire is 1.2 mms. The annular passage area for the mercury is therefore 0.3 sq.mm. These data lead to the following remarks:
when dealing with ducts of great length (e.g. from 3 to 15 meters) for which compensation is of special interest, it is difficult to introduce a fine wire into a tube of such a small cross-section;
dimensional tolerances are difficult to comply with when dealing with such small diameters and even variations of a few hundredths of a millimeter can considerably vitiate the compensation;
the assembly of the tube (outer diameter of about 4 mms.) and the compensation core is stiffer and less resistant to deformations than a capillary by itself;
the deformations of the duct, which cannot be avoided when fitting the apparatus in stationary units, and even more in movable units, produce variations in tube cross-section and hence in liquid volume, which irretrievably results in false indications of the measured temperature.
The present invention is directed to a liquid expansion thermometer which is based on the of the above-noted type, i.e., one which includes a bulb locatable within the medium whose temperature is to be measured, a measuring apparatus and a duct connecting the bulb to the measuring apparatus an expansible liquid filling the assembly, and which is constructed such that temperature compensation for the liquid in the duct can be achieved in a way which allows the duct to retain its flexibility and which allows the thermometer as a whole to be simply constructed.